TwinCAT Zig ADS Server

A lightweight ADS (Automation Device Specification) server written in Zig, capable of communicating with Beckhoff TwinCAT systems over the AMS/TCP protocol.

Project Overview

This project is perfect for:

• Simulating PLC behavior for testing and development
• Building custom device emulators with precise control
• Creating TwinCAT-compatible services for seamless integration

Key Features

• ADS over TCP implementation
• Supports essential ADS commands, including:
  • ReadDeviceInfo
  • Read
  • Write
  • ReadWrite
  • ReadState
  • WriteControl
  • AddDeviceNotification / DeleteDeviceNotification
• Multi-threaded server loop for concurrent client handling
• TwinCAT Router handshake via port 48898
• Modular design with extensible callbacks:
  • OnRead
  • OnWrite
  • OnReadWrite

🧭 Why It Stands Out

• Clean architecture that’s easy to extend and maintain
• very high performance
• Minimal, embeddable design — suitable for integration in custom Zig applications

Usage

Here’s a simple example of how to start the ADS server using Zig:

const std = @import("std");
const Server = @import("server.zig").Server;
const ads = @import("ads.zig");

pub fn main() !void {
    var allocator = std.heap.page_allocator;
    var server = Server.init(allocator, 10000);

    // Override the OnRead callback here:
    server.OnRead = customOnRead;

    try server.start();
    defer server.stop();
}

fn OnRead(IndexGroup: u32, IndexOffset: u32, readData: []u8) !ads.ErrorCode {
    std.log.info("Custom OnRead: IndexGroup={}, IndexOffset={}, len={}", .{IndexGroup, IndexOffset, readData.len});

    if (IndexGroup == 0x4020 and IndexOffset == 0) {
        if (readData.len >= 4) {
            const value: u32 = 0x12345678;
            std.mem.writeIntLittle(u32, readData[0..4], value);
            return ads.ErrorCode.NoError;
        } else {
            return ads.ErrorCode.DeviceInvalidSize;
        }
    }

Browse Zig API Docs

⚠️ Current Limitations

• WriteControl commands are not supported yet.
• AddDeviceNotification and DeleteDeviceNotification commands are currently unimplemented.
• Symbolic access (access via symbol names instead of index groups/offsets) is not supported at this time.
• Ads Router is not integrated, maybe there will be a standalone version soon.
• Ads Client is not integrated,....

These features may be added in future releases. Contributions and feature requests are welcome!

    return ads.ErrorCode.DeviceInvalidParameter;